Update check in `isSSR` function to use asset manifest

[ganemone]

We are currently using a regular expression to match javascript files as a hack to allow viewing them in the browser. We should instead check against the asset manifest so we know exactly what file is being requested

[lhorie]

Related issue: #116 - merely checking asset manifest will not be sufficient to handle situations where the asset is dynamically generated or obtained from a service. A common example is PDF downloads.

[mlmorg]

Won't do this. Will do path-based heuristic for SSR.

[old-fusion-bot]

This issue was automatically re-opened because it is referenced in a TODO.